Hunting For Slabs on a Level 1 Avalanche Course

Hunting for slabs on a Level 1 Avalanche Course – Matt Wade – Peak Mountain Guides The avalanche bulletin for the Northern San Juans read “Recent winds have formed tender slabs on lee slopes near and above treeline. These wind slabs have developed on a very weak and fragile snow structure.”  As we were beginning the Level 1 Avalanche Course it was apparent the snowpack was shallow and variable, and it would require keen observation skills to travel and ski safely. After an introductory day of classroom sessions and a field session to learn companion rescue, we started our second day with a specific goal to learn more about the snowpack in the Northern San Juans.  In particular, we wanted to learn more about the primary avalanche concern that was mentioned in the bulletin – wind slabs – as well as a secondary concern that was mentioned – persistent slabs.  We toured into an area near treeline, on a Northeast aspect, to dig a snow profile for the purpose of observing the layers in the snowpack.  We dug several snow profiles and the students learned how to identify layers.  The students then learned how to assess the hardness of the layers using a hand hardness test and we began our hunt for slabs.  Our ultimate goal was to find the best skiing possible, while avoiding slabs mentioned in the bulletin. We discovered that the snowpack in our profile had no slabs.  In fact, it was entirely faceted top to bottom with no strong over weak layering present.  We performed several compression tests and we got no results (CTN x 3).  As we considered the prolonged cold weather, the shallow snowpack, and the lack of recent storms, it became clear that the faceting process had been hard at work in this cold, sheltered location.   Also important, we identified the average height of the snowpack in this area was 100 cm.  This snowpack height would serve as our baseline for understanding changes in other locations we might visit. With no notable slab avalanche hazard in this area and lots of soft snow to ride, we clicked in and made some turns!  Good turns! After gathering baseline information about the snowpack in a neutral location that had not felt the effects of wind or sun, we were well prepared to understand any changes that appeared in other locations.  For our last day of the course we decided to pursue a ski descent called “Powerline”, a Northwest facing run at treeline that turns into a long tree alley below.  Since our objective was higher in elevation and more subject to wind, it would be critical for us to observe whether or not slabs had developed in this area.  Our main tool for this investigation would be our BCA avalanche probes.  By probing through the new terrain we could measure changes in the height of snow, with any increase in height over our baseline snowpack being an indicator that either persistent slabs and/or wind slabs were present.  We also used the probe to “feel” the layers for areas of stiff snow.  While touring through the initial forest we found the height of snow was consistently around 80 cm deep and it felt the same as the faceted snowpack we observed the previous day.  Using just our probes while traveling we were able to confirm this snowpack presented no significant avalanche hazard and would offer some good, safe skiing. As we left the forest near treeline our probing revealed a change.  We started to find isolated pockets where the snowpack was deeper, up to 150 cm deep.  In these areas we could feel distinctive changes in the stiffness of the snow cover.  We had found the slabs. We traveled over a variety of terrain near treeline, probing along the way.  We learned that the slabs were actually confined to isolated terrain features such as gullies and small bowls.  In most places the persistent slab did not exist, and the wind slab was typically just 2-6 inches thick.  As we gained more information it became apparent that the slab avalanche hazard was very isolated and we would be able to avoid it if we steered clear of gullies and other features that had trapped windblown snow.  When we finally arrived on top of Powerline, we had a good handle on the avalanche problem and how to avoid it.  Looking at our proposed descent we were confident we could find a nice ride while avoiding avalanche hazard. We slid into the top of Powerline and made a few more investigations.  We probed around and found no persistent slab, just a bit of isolated wind slab in small pockets.  We dug a quick profile to evaluate the wind slab and found it was quite reactive, but very discontinuous. This was consistent with our observations while traveling through the terrain on our skis.  We would readily crack the windslab underfoot, but it was so discontinuous that pockets were only 3-4 feet in area with the slab quickly thinning on the edges.  As long as we avoided pockets where the slab could be continuous, such as gullies and bowls, we were good to go. We found a good line on a small rib and rode wind effected snow to the treeline.  From there, we skied below treeline into long stretches of smooth riding on facets.  It was a nice descent and we all enjoyed the ride.  And, we all felt good about our terrain choices.  We were able to make good decisions due to our concerted effort to identify the avalanche problem and its distribution, and then we ultimately chose areas that avoided the problem.  It made for some good learning in our level 1 avalanche course (and some good skiing too!).